Add plugin host architecture and Storm Sewer add-on package

Generic plugin runtime (src/plugin/, HostSession, per-tab plugin_state, command router). QGIS-style add-on layout with plugin.toml and single PluginRegistration. Storm Sewer refactored as reference consumer with dispatch.rs, catchments, LandXML, headless tests (43 passing). Core update.rs decoupled from storm_sewer via CadCommand object-pick hooks. Docs: plugin-architecture.md, plugin-template, PR and issue-78 reply drafts.

docs/plugin-template/PLUGIN.md

@@ -0,0 +1,11 @@
+# My Plugin (`opencad.my_plugin`)
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `MP_HELLO` | Example command |
+
+## XDATA
+
+Document entity record layouts here when your plugin tags geometry.